In communications systems having geographically fixed stations and mobile stations, it is necessary to handoff the communications with the mobile station from a first fixed station to a second fixed station, as the mobile station moves farther away from the first fixed station and closer to the second fixed station. In the cellular communications context, as a mobile station travels out of a first cell and into a second cell, communications with the mobile station must be passed from a base station in the first cell to the base station in the second. Similarly, in the case of base stations provided on orbiting satellites, the position of the "mobile" unit on or near the surface of the Earth is relatively fixed compared to the orbiting satellites, and communications with this unit must be passed from a first base station to a second base station. The second base station can either be on a second satellite or can be associated with a second antennae on the first satellite which services a second beam (geographic region). Hereinafter, base station refers to a fixed base station on the ground or a base station provided on board an orbiting satellite.
There are three types of conventional handoff, depending on the multiple access system employed in the communications system: hard handoff, soft handoff, and softer handoff.
A hard handoff is characterized by a temporary disconnection of the forward and reverse channels and is typical in an FDMA or TDMA environment. As the mobile unit moves out of a currently serviced cell area or satellite beam during a communications session, the received signal becomes weak and a handoff is required. To perform the hard handoff, the communications system switches the communications session to a new channel while the session continues. In hard handoffs in FDMA or TDMA (or one CDMA system to another CDMA system using different frequency spectrums), the receiver in the mobile unit stops demodulating and decoding information transmitted on the old channel link, from the cell or satellite link initially servicing the session, and then starts demodulating and decoding information transmitted via a second channel link.
In a conventional FDMA or TDMA cellular system, the handoff scheme implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between cells. Then handoff from one cell to another is initiated when the receiver in the cell base station handling the call notices that the received signal strength from the mobile station falls below a predetermined threshold value. A low signal to noise ratio indication implies that the mobile telephone is on the cell boundary. When the signal level falls below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station received the mobile signal with better signal strength than the current base station. The system controller in response to the current base station's inquiry sends messages to the neighboring base stations with a handoff request. The base stations neighboring the current base station employ scanning receivers which receive the signal from the mobile station on the specified channel.
A handoff will be attempted once one of the neighboring base stations reports an adequate signal level to the system controller. This scenario is called a "base station initiated handoff process." Handoff is then initiated when an idle channel from the channel set used in the new base station is selected. A control message is sent to the mobile station commanding it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first base station to the selected base station. In the conventional FDMA or TDMA system, a call will be dropped if the handoff of the new base station is unsuccessful. There are several reasons that a failure in handoff may occur.
For example, a handoff can fail if there is no idle channel available in the neighboring cells with proper signal strength. PA1 a handoff can also fail if another base station reports hearing the mobile station in question, when in fact, the base station actually hears a different mobile station using the same channel in a completely different cell. PA1 1) The mobile station served by base station A scans and measures potential pilot signals for two or more base stations. PA1 2) The mobile receiver detects the pilot signal from base station B which exceeds a predetermined threshold level. PA1 3) The mobile station sends a pilot strength measurement message to base station A. PA1 4) Base Station A receives the pilot strength measurement message and relays the message to base station B through the Master Switching Center. PA1 5) Base station B begins transmitting the same traffic for the particular mobile station on the forward channel as that transmitted by base station A and acquires the reverse traffic channel from the mobile station. PA1 6) Base stations A and B each send a handoff direction message to the mobile station to start demodulating signals from A and B. PA1 7) The mobile station receives the handoff direction messages, acquires and demodulates the signal from base station B and begins diversity combining the signals from base stations A and B. PA1 8) The mobile station sends a message to both base stations A and B informing both base stations that it is receiving signals from them both. PA1 9) If the handoff Drop Timer expires as to the pilot signal from base station A, the mobile station sends a pilot strength measurement message to base stations A and B. If a signal from a base station remains below a predetermined threshold value for a predetermined amount of time (i.e., the period of the handoff Drop Timer), the signal from that base station will be dropped from the set of signals being processed by the mobile station as described in steps 10-13. PA1 10) The base stations A and B send a handoff direction message to use only B to the mobile station. PA1 11) The mobile station sends the handoff completion message to the base stations A and B informing the base stations that it will stop receiving signals from base station A. PA1 12) The mobile station receives the handoff direction message, stops diversity combining and begins demodulating signals from base station B only. PA1 13) Having received the handoff completion message, base station A stops transmitting on the forward traffic channel of the mobile station and receiving on the reverse traffic channel. PA1 Improved link quality. Cell boundaries (as used herein, "cell boundaries" also include beam boundaries for satellite systems and cell refers both to the coverage of a ground base station antenna or of a satellite beam) usually offer poor coverage coupled with increased interference from other cells. Therefore, forward traffic channel diversity from additional cells or satellites will improve link quality. PA1 Controlled interference. A mobile unit consists of a mobile station for the cellular environment and a relatively fixed ground terminal from the satellite environment. While on a cell boundary, the mobile unit's interference to mobile units in other cells is maximal. The soft handoff enhances the ability to control the signal power of the mobile unit from these cells, thereby minimizing such interference. PA1 Reduce call dropping probabilities. The forward link is most vulnerable in handoff areas. A slow handoff process coupled with a vehicle moving at a high speed or a fast moving satellite may cause the call to be dropped if the mobile unit is no longer able to demodulate the forward link transmitted from the original cell, thereby losing the handoff direction commands.
For the base station initiated hard handoffs occurring near cell boundaries, signal levels tend to fluctuate at both base stations. This signal level fluctuation gives rise to a "ping-ponging" situation whereby the mobile station is repeatedly instructed by an original base station to handoff the call to a neighboring base station, and then the neighboring base station instructs the mobile station to handoff back to the original base station.
This process is sometimes called "break before connect." Because a hard handoff is completed by a temporary disconnection of the traffic channel, information in the received signal may be lost.
The soft handoff (as used in a CDMA environment) alleviates the problem of the temporary disconnection. In a soft handoff, two or more received signals through different cells or satellites are simultaneously demodulated, combined, and decoded by the same receiver unit. Because the CDMA environment enables the receiver to simultaneously demodulate, combine and decode signals from more than one base station, the soft handoff does not require any disconnection of the traffic channels. A user moving into the service area of another base station or satellite beam does not need to change its receiving or transmitting frequency. A soft handoff is characterized by initiating communications using a new code sequence (i.e., with a new base station at a new cell or satellite) on the same CDMA frequency before terminating communications with the old code sequence.
One soft handoff system used in conjunction with a cellular communication system is described in U.S. Pat. No. 5,640,414 for the "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communications Systems," issued to Blakeney II, et al. (the "Blakeney patent"), which is hereby incorporated herein by reference. The initiation of the handoff process is invoked by the mobile station measuring the signal power of the handoff-assisting pilot signal over the pilot channel in the CDMA system or by the base station measuring the signal power from the mobile station. A typical CDMA soft handoff is implemented by diversity combining (i.e., combining signals from either the same or different base stations) in conjunction with a RAKE receiver, thereby providing better call reliability than a hard handoff and supporting the handoff process between cells or beams in a manner that is transparent to the user.
As described in the Blakeney patent with reference to a cellular communications system, the mobile initiated handoff method is different from the base station initiated handoff method. The mobile initiated handoff relies on the mobile station to detect the presence or absence of pilot signals and the signal strength of the pilot signals. Thus, in order to perform a handoff initiated by a mobile station, the mobile station is equipped with a search receiver to scan pilot signals from other base stations. One reason to employ a mobile initiated handoff method is that the mobile station is more sensitive than base stations to changes in path between itself and various neighboring base stations.
In a conventional CDMA system, two types of handoff operations are implemented; the soft handoff and CDMA-to-CDMA hard handoff. The CDMA-to-CDMA hard handoff is similar to that of the TDMA or FDMA system, and call interruption may occur. It may be helpful in understanding the problems with existing systems to consider a CDMA and its soft handoff procedures in somewhat more detail.
In the soft handoff situation, the mobile station initiates the handoff process. The mobile station performs the signal diversity combining to/from multiple base stations. The mobile station employs RAKE receivers to receive communications simultaneously from the multiple base stations. A soft handoff occurs when the mobile station is communicating simultaneously with two or more base stations or with two or more sectors of the same base station before communications with the previous base station or sector is dropped. The latter case (i.e., between sectors within a cell) is called a "softer handoff". This is a special type of soft handoff, and no distinction is made herein between a soft and a softer handoff. In the soft handoff environment, the call between a mobile station and an end user is not interrupted by the eventual handoff from the base station corresponding to the cell from which the mobile station currently is being serviced to the base station corresponding to the cell from which the mobile station is to receive services.
FIGS. 1-3 depict a conventional CDMA system. As shown in FIG. 1, the diversity combiner receiver of the CDMA system at the mobile receiver includes a diplexer feeding a front end analog receiver 101, which supplies signals to multiple digital RAKE receivers 102A, 102B, 102C and to a searcher receiver 103. The receivers provide data to a diversity combiner 104. The output of the diversity combiner is connected to a decoder 105. If the mobile unit provides telephone service, the decoder supplies signals through base band processing circuitry and a vocoder, to provide signals to drive the handset speaker.
As shown in FIG. 2, the diversity combiner receiver of the conventional CDMA system at the base station has the same configuration as the mobile station except for the diplexer and the number of front end antennae and receivers. At the base station, two receiver systems are used for antenna diversity reception. These two systems independently receive the same CDMA signals and are combined at the diversity combiner 204. Thus, antennae 200A, 200B separately receive a signal from the mobile station, and each antenna supplies the signal to an analog receiver 201. The analog receiver is followed by multiple RAKE receivers 202A and 202B and a searcher receiver 203. The RAKE receivers 202A, 202B output the signals to a diversity combiner 204. Like the output of the diversity combiner 104 in the mobile station, the signal is then decoded in decoder 205. The base station forwards the decoded reverse channel information over a digital link to a master switching center (MSC).
FIG. 3 depicts the operation of the diversity combiner 104 or 204 in either the mobile station or the base station. The diversity combiner utilizes a maximal ratio combiner. The combiner first applies a specific weighted-signal-to-noise-ratio at 301A, 301B, 301C (which is based on their measured signal strength) to the incoming data signals from the individual receivers, here represented generically by receivers 302A, 302B, 302C. The diversity combiner then combines these weighted signals in the adder 303. The diversity combining scheme is termed "a maximal ratio combiner." The combining is coherent, since pilot signal demodulation allows aligning of multiple streams of received signals. The resulting combined signal is then decoded by decoder 304 using forward error correction. The conventional forward error correction uses the convolutional code with an appropriate Viterbi algorithm decoder. An exemplary conventional CDMA cellular system uses convolutional codes. Such a system has a code rate of 1/2 for the forward link from a base station to a mobile station and a code rate of 1/3 for the reverse link from a mobile station to the base station.
The conventional call processing operations during a soft handoff from base station A to base station B include the following:
The mobile station initiated handoff method provides more reliable handoff and increased system performance than the base station initiated handoff. Diversity combining at the mobile station receiver of signals from multiple base stations in conjunction with the RAKE receiver allows the mobile receiver to receive multiple copies of the same CDMA signal from different base stations and multiple copies of the same signal from each base station.
The softer handoff, which is one special case of the soft handoff operation, occurs when a mobile station is moving from one sector coverage to another sector coverage in the same CDMA cell. The softer handoff uses the same procedures of the soft handoff except that the softer handoff occurs between sector antennae of the same base station instead of between base stations.
There are at least three reasons why the soft handoff is preferable over the hard handoff if system design allows.
While each of the above described systems provide for handoffs of cellular calls, none of the systems provide handoffs that are as reliable as either a communications system provider or a communications system user would prefer. Common problems continue to occur in the handoff region of a given cell or satellite beam, including interference, fading (excessive path loss) or echoing (time delay spread phenomenon) of the signal, and multipath fading. Interference is caused by signals from neighboring cells. The mobile station may inadvertently interpret a signal from a neighboring cell or satellite and process the signal as through it was the intended signal to be received. Thus, it is possible that a user at a mobile station is interjected into a wholly unrelated communications session. In addition, the signal may fade as the distance grows between the mobile station and the base station. The distance between the transmitting end and the receiving end of a signal, combined with buildings and the topography of the surrounding terrain, may also cause the signal to be disrupted and, thus, faded. The multitude of signals that eventually reach the receiver may also have traveled via different paths from the transmitter. Because the path lengths are different, the signals will not arrive at the receiver at the same time. Thus, the receiver may process two different versions of the same information, causing frequency selective fading.
Many of these problems can be mitigated by channel coding the signals used to communicate with the mobile station. Present channel coding systems do not differentiate the signal sent from different base stations to the same mobile station as a handoff occurs. As discussed above, conventional CDMA cellular systems utilize multiple receivers to detect multipath signals and/or signals from different base stations. These signals are time delayed versions of the same coded signal and can be combined by a RAKE receiver and a diversity combiner. The conventional diversity combiner uses the maximal ratio combiner on the "same" signals received from different base stations, from different sectors or on different multipath signals. The receiver thus receives the same exact signal from both base stations during a handoff operation. Because the signals received from both base stations in the handoff region are the same coded signal, the amount of gain is limited to only the diversity combining gain and the designed coding gain.
Recently, new coding techniques have enabled communications systems designers to achieve greater coding gains. For example, the iterative coding technique described in U.S. Pat. No. 5,446,747 for an "Error-Correction Coding Method with at Least Two Systematic Conventional Codings in Parallel, Correspondinc Iterative Decoding Method, Decoding Module and Decoder" issued to Claude Berrou (the "Berrou patent"), provides substantial coding gains compared with the conventional CDMA system whose coding gain on the forward link and return link are 1/2 and 1/3, respectively. The disclosure in the Berrou patent is hereby incorporated herein by reference. Such a coding scheme as described in the Berrou patent is commonly known as "turbo coding." Generally, turbo coding allows a single signal to be encoded in multiple manners for simultaneous transmission. Thus, multiple coded versions of the single signal can be received and combined to achieve increased coding gain. Moreover, a receiver that receives only one of the coded versions of the single signal still has enough information about the signal to adequately decode it.
To date, there have not been any communications systems which have taken advantage of turbo coding to enhance the systems performance in a handoff region and during the handoff operation. The Berrou patent suggests turbo coding only for a Gausian channel, not for a channel subject to fading. As a result, Berrou transmits only the information signal and two parity bit signals.